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Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest level of the planet's atmosphere, the troposphere, just below the stratosphere. Weather refers to day-to-day temperature and precipitation activity, whereas climate is the term for the averaging of atmospheric conditions over longer periods of time. When used without qualification, "weather" is generally understood to mean the weather of Earth.

Weather is driven by air pressure, temperature, and moisture differences between one place and another. These differences can occur due to the Sun's angle at any particular spot, which varies with latitude. The strong temperature contrast between polar and tropical air gives rise to the largest scale atmospheric circulations: the Hadley cell, the Ferrel cell, the polar cell, and the jet stream. Weather systems in the middle latitudes, such as extratropical cyclones, are caused by instabilities of the jet streamflow. Because Earth's axis is tilted relative to its orbital plane (called the ecliptic), sunlight is incident at different angles at different times of the year. On Earth's surface, temperatures usually range ±40 °C (−40 °F to 100 °F) annually. Over thousands of years, changes in Earth's orbit can affect the amount and distribution of solar energy received by Earth, thus influencing long-term climate and global climate change.

Surface temperature differences in turn cause pressure differences. Higher altitudes are cooler than lower altitudes, as most atmospheric heating is due to contact with the Earth's surface while radiative losses to space are mostly constant. Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. Earth's weather system is a chaotic system; as a result, small changes to one part of the system can grow to have large effects on the system as a whole. Human attempts to control the weather have occurred throughout history, and there is evidence that human activities such as agriculture and industry have modified weather patterns

Studying how the weather works on other planets has been helpful in understanding how weather works on Earth. A famous landmark in the Solar System, Jupiter's Great Red Spot, is an Anticyclonic storm known to have existed for at least 300 years. However, the weather is not limited to planetary bodies. A star's corona is constantly being lost to space, creating what is essentially a very thin atmosphere throughout the Solar System. The movement of mass ejected from the Sun is known as the solar wind. (Full article...)

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Hurricane Katrina Eye viewed from Hurricane Hunter.jpg

View of the eyewall of Hurricane Katrina taken on August 28, 2005, as seen from a NOAA WP-3D Orion hurricane hunter aircraft before the storm made landfall on the United States Gulf Coast.

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Average annual temperatures across India: Lavender is Below 20.0 °C (< 68.0 °F), blue is 20.0–22.5 °C (68.0–72.5 °F), green is 22.5–25.0 °C (72.5–77.0 °F), yellow is 25.0–27.5 °C (77.0–81.5 °F), and red is Above 27.5 °C (> 81.5 °F)

The climate of India is composed of a wide range of weather conditions across a large geographic scale and varied topography. Analyzed according to the Köppen system, India hosts six major climatic subtypes, ranging from desert in the west, to alpine tundra and glaciers in the north, to humid tropical regions supporting rainforests in the southwest and the island territories. Many regions have starkly different microclimates. The nation has four seasons: winter (January and February), summer (March to May), a monsoon season (June to September), and a post-monsoon period (October to December).

India's unique geography and geology strongly influence its climate; this is particularly true of the Himalayas in the north and the Thar Desert in the northwest. The Himalayas act as a barrier to the frigid katabatic winds flowing down from Central Asia. Thus, North India is kept warm or only mildly cold during winter; in summer, the same phenomenon makes India relatively hot. Although the Tropic of Cancer—the boundary between the tropics and subtropics—passes through the middle of India, the whole country is considered to be tropical.

As in much of the tropics, monsoonal and other weather conditions in India are unstable: major droughts, floods, cyclones and other natural disasters are sporadic, killing or displacing millions. India's long-term climatic stability is further threatened by global warming. Climatic diversity in India makes the analysis of these issues complex.

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Did you know...

...that a hurricane force wind warning is issued by the United States National Weather Service for storms that are not tropical cyclones but are expected to produce hurricane-force winds (65 knots (75 mph; 120 km/h) or higher)?

...that the Automated Tropical Cyclone Forecasting System is a software package for tropical cyclone forecasting developed in 1988 that is still used today by meteorologists in various branches of the US Government?

...that a cryoseism is a sudden ground or glacier movement that can occur due to water freezing or ice cracking after drastic temperature changes?

...that BUFR is a binary data format standardized by the World Meteorological Organization for storing observation data from weather stations and weather satellites?

...that the Northwest Weather and Avalanche Center issues weather forecasts for conditions that can cause avalanches in the mountains of western Washington and northwestern Oregon?

...that a wind chill warning is issued by the National Weather Service when a combination of wind and cold temperatures is expected to cause life-threatening conditions for anyone caught outside?

Recent and ongoing weather

This week in weather history...

June 8

1953: On the second day of a deadly three-day tornado outbreak, an F5 tornado killed 116 people in Flint, Michigan.

June 9

1953: On the final day of a deadly three-day tornado outbreak, a violent tornado plowed through the city of Worcester, Massachusetts and surrounding towns, killing 90 people.

June 10

2005: A severe flood struck the town of Shalan in the Heilongjiang province of China, killing more than 100 people.

June 11

1979: A tropical depression formed over the Caribbean Sea south of Grand Cayman. The storm produced torrential rainfall over Jamaica, including up to 32 inches (810 mm) in Westmoreland Parish.

June 12

2004: Tropical Storm Chanthu made landfall in central Vietnam, killing 39 people.

June 13

1968: Hurricane Abby dissipated east of Virginia.

June 14

1903: A major flash flood destroyed a large portion of the town of Heppner, Oregon, killing 247 people. This remains the deadliest natural disaster in the history of the state of Oregon.

Selected biography

Portrait of Sir George Gabriel Stokes

Sir George Gabriel Stokes, 1st Baronet, PRS (/stks/; 13 August 1819 – 1 February 1903) was an Anglo-Irish physicist and mathematician. Born in County Sligo, Ireland, Stokes spent all of his career at the University of Cambridge, where he was the Lucasian Professor of Mathematics from 1849 until his death in 1903. As a physicist, Stokes made seminal contributions to fluid mechanics, including the Navier–Stokes equations and to physical optics, with notable works on polarization and fluorescence. As a mathematician, he popularised "Stokes' theorem" in vector calculus and contributed to the theory of asymptotic expansions. Stokes, along with Felix Hoppe-Seyler, first demonstrated the oxygen transport function of hemoglobin and showed color changes produced by aeration of hemoglobin solutions.

Stokes was made a baronet (hereditary knight) by the British monarch in 1889. In 1893 he received the Royal Society's Copley Medal, then the most prestigious scientific prize in the world, "for his researches and discoveries in physical science". He represented Cambridge University in the British House of Commons from 1887 to 1892, sitting as a Conservative. Stokes also served as president of the Royal Society from 1885 to 1890 and was briefly the Master of Pembroke College, Cambridge. (Full article...)

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WikiProject Meteorology is a collaborative effort by dozens of Wikipedians to improve the quality of meteorology- and weather-related articles. If you would like to help, visit the project talk page, and see what needs doing.

WikiProject Severe weather is a similar project specific to articles about severe weather. Their talk page is located here.

WikiProject Tropical cyclones is a daughter project of WikiProject meteorology. The dozens of semi-active members and several full-time members focus on improving Wikipdia's coverage of tropical cyclones.

WikiProject Non-tropical storms is a collaborative project to improve articles related to winter storms, wind storms, and extratropical cyclones.

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